The completion of an oil, gas, water or geothermal well often involves hydraulic fracturing wherein spacer particles (called proppants) are injected with the fracturing fluid to maintain the fractures in a propped condition. While sand is the most common proppant because of its low cost and availability, other particles have also been used such as glass beads as taught in U.S. Pat. No. 3,155,162 (Flickinger et al.). According to U.S. Pat. No. 3,399,727 (Graham et al.), the proppant theoretically should be distributed within each fracture as a partial monolayer, leaving relatively large spaces between its individual particles through which the fluid being recovered can flow. However, if its density greatly exceeds that of the fracturing fluid, the proppant tends to settle out, leaving upper portions of fractures poorly supported. The Graham patent deals with this problem by using as the proppant glass or other ceramic particles having internal voids and hence a density approaching that of the fracturing fluid. The Graham patent also suggests increasing the density of the fracturing fluid by the addition of a weighting agent such as BaSO.sub.4, BaO or ZnS.
Although a great many other patents concern improved proppants, no other prior art has been found which discloses a proppant of reduced density other than particles having internal voids.
Among patents dealing with improved proppants is U.S. Pat. No. 3,026,938 (Glenshaw et al.) involving coating the entire surface of each particle of a proppant with a nonbrittle plastic film to confine and prevent dispersion of small fragments should the particle rupture under the pressure of a propped fracture. U.S. Pat. No. 3,929,191 (Graham et al.) discloses additional coating resins such as phenol formaldehyde resins. The proppant of U.S. Pat. No. 3,376,930 (Kiel et al.) consists of individual frangible cores, to each of which is adhesively bonded a relatively thin coating of metallic microparticles. U.S. Pat. No. 4,078,610 (Arnold) employs metal-plated proppant particles coated with a thermoplastic polymer such as polyethylene. The proppant of U.S. Pat. No. 4,231,428 (Needham et al.) has an organo-silicon coating to protect the cores of the particles from water which otherwise might reduce their compressive strength.